Semiconductor devices using a semiconductor integrated circuit technique, such as charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS) image sensors, are widely used in digital cameras and portable phones with camera functions. In response to miniaturization and weight reduction of parts to be mounted on such a semiconductor device, a technique is proposed in which a sensor chip (semiconductor element) is packaged in a chip size package (CSP). A method for packaging a semiconductor element such as a sensor chip in the CSP is to provide a semiconductor substrate having sensor devices or the like formed thereon with a through electrode that connects the front and back surfaces of the semiconductor substrate (see, the pamphlet of International Patent Publication No. WO2005/022631).
The semiconductor device as described above is manufactured as follows, for example. Firstly, a through hole is formed in a semiconductor substrate on which a front insulating film and a front wiring layer are formed in this order so as to expose the front insulating film through the through hole. Then, a back insulating film is formed on the inner wall surface of the through hole and the back surface of the semiconductor substrate. Thereafter, the front and back insulating films are etched so that the front wiring layer can be exposed to the inside of the through hole. Then, a through wiring layer is formed to extend from the inside of the through hole to the back surface of the semiconductor substrate so as to be connected to the front wiring layer via the through hole. After that, an external connection terminal is formed on the through wiring layer. In this way, multiple semiconductor devices as described above are formed on a semiconductor wafer, which is eventually cut into pieces.
The insulating film formed on the inner wall surface of the through hole is desired to have leakage resistance properties comparable to a thermally-oxidized film. However, it is difficult to form an insulating film by using such a high-temperature process as used to form a thermally-oxidized film, if the insulating film is to be formed on a semiconductor substrate on which an integrated circuit has been already formed. Moreover, an image sensor has a microlens array and a color filter which are made of an organic material and formed on a surface of the image sensor, and thus an allowable temperature at formation of the insulating film is around 200° C. Accordingly, as the insulating film in the through hole, used is an SiO2 film formed by a low-temperature formation technique such as a CVD method.
However, if the insulating film is an SiO2 film formed by a low-temperature formation technique, the insulating film tends to be thin at the corner between the sidewall and the bottom of the through hole. This might lead to current leakage at the interface between the semiconductor substrate and the through wiring layer, and thus to malfunctions of the sensor device or the like. This eventually reduces a manufacturing yield and electrical reliability of the semiconductor device. Moreover, stress concentration is likely to occur at the corner of the bottom of the through hole. Accordingly, if such a thin insulating film is employed, cracking might occur therein, which reduces reliability of the semiconductor device. In order to increase the thickness of the insulating film at the corner, the entire insulating film on the sidewall may be formed with an increased thickness. However, the insulating film in which the entire area is increased in thickness cannot be applied on the through hole having a small diameter.